Hydraulic actuators of the type designed to be hydraulically independent are well known in the aerospace field and are used for a variety of applications such as to provide vectoring of a rocket's exhaust nozzle or to move control surfaces aboard an aircraft. These actuators generally comprise a hydraulic pump, a linear or rotary actuator that converts the pressure energy of the pumped hydraulic fluid into mechanical energy in the form of linear or rotary motion, and means for driving the hydraulic pump, and generally come in two configurations. The first configuration is the hydrostatic actuator in which the hydraulic pump is in direct fluid communication with the actuator. This system is static because, except for the pumping required to make up for leakage flow in the system, the hydraulic pump only operates when a command to move the actuator is received. The second configuration is the recirculating hydraulic actuator in which a servo-valve is used to control the flow of hydraulic fluid from the hydraulic pump to the actuator and from the actuator to a reservoir. Also, excess flow from the pump is dumped through a relief valve into the reservoir. In this recirculating configuration the hydraulic pump operates continuously, circulating the hydraulic fluid between itself and the reservoir. Only when actuation is required is hydraulic fluid sent to the actuator.
In the hydrostatic configuration, the hydraulic pump is commonly driven by a brushless electric motor. A detailed description of an electrohydrostatic actuator can be found in Chamberlain, U.S. Pat. No. 4,630,441. The use of electric motors to drive the pump has a number of disadvantages. First, the electric motor requires a turbogenerator and circuitry to supply it with adequate electric current. The use of a turbogenerator and circuitry not only adds weight and electronic noise to the vehicle but also reduces the reliability of the system by adding additional failure modes associated with these components. Furthermore, due to size and weight constraints imposed for aircraft and rocket applications, these electrically driven actuators have been limited to outputs of about 45 horsepower. However, there are some applications on airplanes and rockets that require outputs greater than what electrically driven configurations can provide.
Some vehicles have a readily available source of pressurized gas. In these cases, high pressure ratio, impulse type turbine wheels have been used to drive the hydraulic pump on a recirculating configuration. Pressurized gas is bled from the vehicle's gas supply and expanded across the turbine wheel which converts the pressure energy of the gas into rotary motion that drives the hydraulic pump. This configuration is capable of generating up to about 100 horsepower. A disadvantage to using these turbine wheels is their high inertia. Because of their high inertia, the turbine wheels are very slow in accelerating to operating speed. In modern aircraft and rockets, actuators must be able to respond quickly. As a result, in order to meet this quick response time the turbine wheel must be kept running at full operating speed even when no actuation is required. This constant running generates large amounts of heat which requires an elaborate cooling mechanism to dissipate. Also, because the turbines must run continuously, they cannot be used with hydrostatic systems.
Therefore, where pressurized gas is available, there is a need for a hydraulic actuator that could be driven using the pressurized gas and also, could generate sufficient horsepower, satisfy the fast response times required of the actuator, and not generate large amounts of heat during those periods in which actuation is not required. Further, the hydraulic actuator should be packaged in a unitary structure so that it can be mounted in tight spaces aboard the rocket or aircraft and mounted near the rocket or aircraft member that is to be actuated.